Diphenhydramine is an antihistamine (Benadryl). It works well as a sleep aid because of the drowsiness it causes as a side effect. It's not an NSAID and unfortunately can't explain the ulceration. Bummer.
You're describing your doctor's advice as "skipping" to biologics. This suggests that there's something you believe you should try first, for whatever reasons. Do you know what that is, or are you mostly just hoping something exists? Since you're not really on board with the doctor's diagnosis, it will be hard to come to an agreement about treatment. If you don't want to be treated for Crohn's, but your doctor is convinced of the diagnosis (it sounds like testing is still going on?), then you might even have to switch doctors.
Less drastically, you could ask the doctor what treatment alternatives exist, since you are worried about biologics. You could ask how certain the Crohn's diagnosis is, and ask about trying to treat the next-most-likely alternative. You could consider trying a biologic for a few months to see if it makes you feel better and whether it causes the side effects you're particularly concerned about. You can always stop it if it's not working, or causing side effects you don't want to put up with.
I've live with the possibility of a Crohn's diagnosis for nearly 40 years. At times I was in denial, and at times I went along with my doctor's advice. My J-pouch was, in part, treatment for the next-most-likely alternative to Crohn's. At the time I hoped the pouch would last a few years. I got lucky, and either 1) I don't have Crohn's, or 2) my Crohn's is going easy on me.
Whatever you do next, try to pay attention to how it's really going, and be open to a change (even an unwelcome one) if it turns out to be necessary. Untreated Crohn's is sometimes much worse than weight gain. The folks here with fistulas and such can attest to that.